The Carp That Ate a Lake

Ninety-One Percent

In the early 2000s, biologists weighed up everything swimming in Utah Lake and found that common carp accounted for 91 percent of the entire fish biomass. Not 91 percent of the individual fish. Ninety-one percent of the living weight. Utah's largest freshwater lake had effectively become a carp pond that happened to contain a few other species.

Nobody planned this. Nobody wanted it. And the strangest part of the whole story is that the United States government did it on purpose.

A Government Fish Nobody Would Eat

In the late 1800s, the federal government went looking for a cheap protein source to feed a fast-growing country. Common carp, native to Europe and Asia, looked ideal on paper: hardy, fast-growing, and able to thrive in warm, murky, low-oxygen water that would kill a trout. Fish commissions shipped them across the country and dropped them into lakes and rivers, Utah Lake among them.

The plan had one flaw. Americans took one look at carp and refused to eat it. The fish stayed, the appetite never arrived, and in a warm, shallow, nutrient-rich lake with no predator that could touch an adult carp, the population did what carp populations do.

A century later they owned the place.

What Carp Actually Do to Water

Carp feed by rooting through the bottom, and that habit is the whole problem. They uproot the aquatic plants that everything else depends on for food, cover, and spawning. They churn sediment into the water column, which turns the lake cloudy and blocks the light those plants need to regrow. And the churning releases nutrients locked in the mud, which feeds algal blooms, some of which produce toxins dangerous enough to close the lake to swimmers and dogs.

The result is a self-reinforcing loop. Carp make the water murky, murky water kills the plants, dead plants mean nothing holds the sediment down, and more loose sediment means more carp habitat and fewer places for anything else. Native fish declined. Stocked game fish struggled. The lake got browner every decade.

One species had further to fall than any other.

The Fish Found Nowhere Else on Earth

The June sucker lives in Utah Lake and its tributaries and nowhere else in the world. It gets its name from its spawning run: every May and June, the adults leave the lake and push up the Provo River to breed. They reach 17 to 24 inches and around five pounds, and they fed the Ute people, trappers, and early settlers for generations.

There were millions of them in the 1800s. By the mid-1980s there were fewer than a thousand, and it was listed as endangered in April 1986. By the early 1990s the count of adult fish in Utah Lake had fallen below 300, with essentially no successful reproduction happening in the wild. A species that existed in exactly one lake was down to a number you could fit in a school gym.

Twenty-Nine Million Pounds

In 2010 a coordinated removal effort began, and it is not subtle work. Commercial crews net carp out of Utah Lake by the ton, year after year, at a rate approaching four million pounds annually. Through 2026 the total pulled out of the lake has passed 29 million pounds.

It worked. Carp density is down about 78 percent since 2012, and managers now say the population is finally at a level they can keep in check. Aquatic vegetation has started coming back in places it had not grown in decades.

And the June sucker turned around. On 3 February 2021 it was officially reclassified from endangered to threatened, one of the rare fish in America to move in that direction. More than 3,500 fish now make the annual spawning run up the Provo River and Hobble Creek. It is not saved yet, and federal biologists are careful to say so, but a species that was down to 300 adults is running again.

How It Happened

• 1880sFederal fish commissions stock common carp across the United States as a cheap food fish. Utah Lake gets its share.
• 1986The June sucker is listed as endangered in April. Fewer than a thousand remain.
• 1990sAdult June suckers in Utah Lake fall below 300, with no meaningful natural reproduction.
• 2000sSurveys find carp make up 91 percent of the lake's total fish biomass.
• 2010Large-scale commercial carp removal begins on Utah Lake.
• 2021The June sucker is downlisted from endangered to threatened, effective 3 February.
• Now29 million pounds of carp removed, density down 78 percent, and more than 3,500 suckers running the Provo.

Can You Actually Help? Yes, and You Get Paid

This is the part most people do not know. The Utah Lake Authority runs a tournament series called The Great Carp Hunt, and it is open to ordinary anglers. You are not volunteering for a cleanup. You are getting paid by the fish to go catch carp in a lake that badly needs fewer of them.

So Is It Pointless?

Fair question, and the honest answer is no, but with an asterisk. Carp will never be eradicated from Utah Lake. Nobody serious is trying to. The goal is suppression: hold the population low enough, for long enough, that plants can root, water can clear, and native fish can reproduce on their own. That target has largely been hit, which is why the June sucker moved off the endangered list.

The asterisk is that suppression is permanent work. Stop netting and the carp come back, because a lake this warm and shallow is close to perfect for them. So the fishing tournaments are not a gesture. A season of them removes thousands of fish, and every undersized carp taken out is one that never spawns. It is a small contribution to a very large number, but the number only stays large because people keep adding to it.

If you want your fishing trip to do something besides feed you, this is the one in Utah where the math actually works.

Sources

June Sucker Recovery Implementation Program, Managing Non-Native Fish and Meet the June Sucker.
Utah State University Institute for Land, Water & Air, Reducing Carp Biomass Initiates Beneficial Ecosystem Response.
Daily Herald, Utah Lake's June sucker to be downlisted from endangered to threatened.
Utah Lake Authority, The Great Carp Hunt.